1. Field of the Invention
The invention relates to solenoid operated valves for petroleum production wells and, more particularly, to a structural and power supply arrangement for an electrical solenoid operated safety valve system.
2. History of the Prior Art
Oil and gas wells, and in particular those located off-shore, are frequently subject to wellhead damage which may be produced by violent storms, collisions with ships and numerous other disastrous occurrences. Damage to the wellhead may result in the leakage of hydrocarbons into the atmosphere producing the possibility of both the spillage of the petroleum products into the environment as well as an explosion and fire resulting therefrom. In addition to off-shore production wells, another environment in which damage to a wellhead may have disastrous effects is that of producing wells located in urban areas. Moreover, in such urban production wells, it is generally a specific legal requirement that there be some downhole means of terminating the flow of petroleum products from the well in the event of damage to the well head. In such instances, the safety valve system must be responsive to a dramatic increase in flow rate from the well so as to close down and terminate production flow from the well. For these reasons, sub-surface safety valves located downhole within a borehole have long been included as an integral part of the operating equipment of a petroleum production well.
Various types of petroleum production flow safety valve systems have been provided in the prior art. Each system includes a valve means for controlling the flow of petroleum products up the tubing from a point down in the borehole from the wellhead. Safety valve systems also include sensing means which are responsive to wellhead damage, a dramatic increase in production flow, or some other emergency condition requiring that the flow from the well be terminated by the valve.
One type of operating mechanism used to actuate a safety valve within a well includes an electrical solenoid employed to hold the safety valve in an open condition and a spring means to return it to a normally closed condition in response to interruption in the flow of current to the solenoid. Numerous such systems have been proposed, for example, U.S. Pat. No. 4,002,202 to Huebsch et al, U.S. Pat. No. 4,161,215 to Bourne, Jr. et al, and U.S. Pat. No. 4,566,534 to Going III. Each of these systems provide a solenoid actuated operating mechanism for the safety valve which is responsive to a DC electric current supplied from surface equipment. Such solenoids generally require a fairly high level surge of initial operating current to cause the solenoid to operate and change states and then a smaller level of current to hold the solenoid in its operated condition. These large actuating current surges require heavy electrical conductors in order to carry such current downhole for any substantial distance and still maintain a voltage level sufficient to operate the solenoid. Moreover, such solenoids are usually supplied with current from a conventional power supply at the surface which produces a fixed voltage output signal. This limits the depth to which the solenoid can be used and still operate with a particular power supply configuration. Use of the same solenoid actuated safety valve in deeper wells requires a change in the power supply circuit in order to supply sufficient current to operate it.
Prior art solenoid actuated safety valve systems have also dealt with the design constraints of high downhole pressures and corrosive borehole fluid in a relatively conventional manner. For example, large values of downhole pressure have required that the pressure resisting walls of the parts of the valve isolating the coil from well pressure be relatively thick in order to serve as a load bearing member of the valve assembly and protect the valve components inside. Thick walls both increase the diameter of the overall valve structure for a given pressure rating as well as limit the thickness of the magnetic armature of the valve and, hence, restricts its magnetic responsiveness to a given value of solenoid actuation current. Similarly, prior art solenoid actuated safety valves have also relied upon the precise machining of valve parts and the presence of high pressure resilient seals, such as O-rings, in order to protect the internal electrical components of the valve, such as the solenoid coil, from borehole fluids. Such fluid sealing components increase the cost of the safety valve and are subject to failure under use. The structure and construction techniques of the valve systems of the present invention overcome many of these disadvantages of prior solenoid actuated safety valve systems.
The inherent disadvantages of providing several different power supply circuits for different depths of operation of a solenoid actuated safety valve is obviated by the system of the present invention which provides means for coupling a constant value of current from the surface down the electrically conductive path interconnecting that current to the windings of a solenoid actuated safety valve. The system provides an optimum value of current for actuation of the solenoid and control of the safety valve regardless of the voltage required to deliver that current to the solenoid at the particular depth of the safety valve. In addition, the solenoid actuated safety valve of the present invention also allows construction of a less expensive and more reliable valve which is of a smaller overall diameter for a particular pressure rating of the valve. In addition, the safety of the present invention is more magnetically responsive for a given valve of operating current delivered to the solenoid coil.
The system of the present invention overcomes many of the disadvantages of the prior art electrically operated solenoid actuated safety valve systems.